Flowmeter for airway resistance measurements

ABSTRACT

A flowmeter has a disposable flow tube and a part of the shutter which are in direct contact with exhaled air and protect the rest of the device from potential contamination with harmful viruses.

This application is a continuation of U.S. patent application Ser. No.15/222,396 filed on Jul. 28, 2016 that claims priority of U.S.provisional patent application 62/197,624 filed Jul. 28, 2015, thespecification of which is hereby incorporated by reference.

The present application relates to medical diagnostics devices, moreparticularly to devices that measure respiratory parameters such asairway resistance.

One known method and the device for measuring airway resistance areknown from Applicant's PCT/CA2014/051073 filed on 6 Nov. 2014 andpublished as WO2015/066812 on 14 May 2015. According to this reference,the subject exhales quietly into the flowmeter with distal end initiallyclosed by the shutter. During the occlusion stage, pressure insideclosed flowmeter increases. After pressure exceeds predeterminedpressure, the shutter is opened, and the device measures thepost-occlusion air flow spike. Airway resistance is calculated based ondata on occlusion pressure and air flow waveforms.

The device described in the earlier patent application can operate withone gauge sensor which measures positive pressure with respect toambient during occlusion and negative pressure with respect to ambientcaused by air flow after the shutter release. Due to Bernoulli's effect,drop in pressure (negative pressure relative to ambient) caused byincreased air velocity through restriction in the flow tube pulls in(entrains) air through the port. This entrainment effect can be createdfor example in Venturi or Pitot tube. Traditional calibration processdetermines relation between negative pressure and air flow through thetube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One object of the proposed solutions is to propose a design of theflowmeter with a disposable flow tube and part of the shutter which arein direct contact with exhaled air and protect the rest of the devicefrom potential contamination with harmful viruses.

Another object of some embodiments is to improve performance of theflowmeter by increasing of pressure response versus flow and simplifyinglinearization of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A is a schematic view of the device for airway resistancemeasurements disassembled showing a reusable electro-mechanical module,a disposable flow tube and a shutter lid which can occlude a distal endof the flow tube;

FIG. 1B is a schematic view of the assembled device for airwayresistance measurements showing a reusable electro-mechanical module, adisposable flow tube and a shutter lid occluding a distal end of theflow tube;

FIG. 1C is a schematic view of a variant device for airway resistancemeasurements showing a tongue depressor integrated into a proximal endof the disposable flow tube;

FIG. 2A presents a schematic view of a disposable flow tube with abaffle and its connection to one pressure sensor;

FIG. 2B presents a schematic view of a disposable flow tube with abaffle and its connection to two pressure sensors;

FIG. 3 shows pressure from flow response of the flow tube;

FIG. 4 shows calibration curves of the flow tube presented as flowversus square root of pressure;

FIG. 5 shows calibration curves of the flow tube with different baffles;

FIG. 6 shows pressure noise measured for the flow tube with differentbaffles.

DETAILED DESCRIPTION

Disposable Flow Tube

One of possible embodiments of the respiratory device is illustrated inFIG. 1A and FIG. 1B. The device consists of disposable flow tube 1, theshutter or cap 2 of the shutter and permanent electro-mechanical module.

The seal between the flow tube 1 and the shutter 2 can be improved byproviding a gasket. The shutter 2 and/or the permanentelectro-mechanical module can have a holder or support for thedisposable shutter, such as a frame or hinge mechanism to support thedisposable part while operating as shutter. In the embodimentillustrated in FIG. 1B, the shutter 2 has a plastic spherical middlesurface with a ferromagnetic rim ring that is all disposable.

During the measurements, the flow tube 1 is attached to theelectro-mechanical module such that gauge pressure sensor 3 is inpneumatic connection with the flow tube. Pneumatic port 4 of the tube isconnected to the input of the sensor 3 through the adaptor 5.

To avoid propagation of the viruses which may be present in air flow,filter 6 is attached to the flow tube 1 between inner surface of thetube and pneumatic port 4.

Because the flow rate through the sensor 3 is small in thisconfiguration, the filter can pose relatively high resistance withoutadversely affecting flow or pressure measurements using the sensor 3.

With such configuration of the device, electro-mechanical module whichmay include also micro-controller 7 and shutter mechanism 8 is isolatedfrom exhaled air and protected from contamination.

In the embodiment of FIGS. 1A and 1B, the flow tube 1 can be providedsterile to the patient for single use, while the electro-mechanicalmodule can be reused.

The micro-controller 7 can alternatively comprise a data interface, forexample a Bluetooth wireless link, so that signal collection and/orprocessing can be performed using a separate unit such as a smartphone,tablet or personal computer. This allows for the user interface to beexternal to the reusable module. The permanent electro-mechanical modulecan have a battery that can be rechargeable.

It will be appreciated that the reusable module can include anindependent user interface, such as an audio signal and/or signal lightsand/or a small display, for interacting with the user during trials. Thefinal measurement can be communicated to the user in a variety of ways.In some cases, only a comparative measurement is required, so the resultcan be communicated with a very simple indicator (visual or audio). Textto speech can also be used to communicate a measurement in audio forminstead of a visual display. Result data can also be communicated by adata link, while patient interaction is done using indicators (audio orvisual) on the device itself.

While the shutter release mechanism 8 shown is a magnetic releasemechanism, a user actuated mechanical trigger can be provided. Ifdesired, the micro-controller can signal to an audio transducer an audiosignal to prompt the user to use the trigger mechanism.

The shutter control is illustrated as an electro-magnetic release,however, a mechanical release mechanism using a piezo-electric actuator,electric motor, solenoid, etc. can also be contemplated. A manualrelease can also be used, although with greater need for operatorcooperation and thus a risk for measurement error.

In the embodiment of FIGS. 1A and 1B, the filter is provided before port4 in the body of the tube 1. Alternatively, the filter 6 can be providedat the port 4, with the coupling of the port being possibly larger toaccommodate the filter 6.

In the embodiment of FIGS. 1A and 1B, sensor 3 is a part of the reusablemodule, and the filter 6 acts to protect the sensor from contamination.Two variants to filter 6 are contemplated.

One variant is to measure the volume of air that is able to be drawnfrom sensor 3 into tube 1 during the largest exhalation expected. Thenthe volume of the passage from sensor 3 to the tube 1 can be arranged tobe bigger than what the one or more exhalation trials could transportback from sensor 3 any possible virus or microbe. A labyrinth passagebetween tube 1 and port 4 could be provided. This would replace the needfor the filter 6.

Following quantitative example demonstrates possibility to build suchlabyrinth. Assume that pressure in the flow tube during occlusionmonotonically increases from zero to 1000 Pa in 1 second. Calorimetrictype thermal sensor with pneumatic impedance of 50 kPa*s/ml is used tomeasure said pressure. Volume of air leaking through the sensor duringocclusion is about 0.01 ml. If the capillary with cross section of 2×2mm is built around the flow tube with outer diameter of 2 cm, its volumeis 0.25 ml. Even one loop of such capillary having volume 25 timesbigger than the volume leaking from the flow tube during occlusion canprotect the sensor from contamination. Note that during post-occlusioncycle, air flows in opposite direction—from the sensor into the flowtube.

A second variant is to arrange the sensor 3 in the disposable tube 1 andto provide an electrical connection between the reusable module and thedisposable tube 1. In this case, the filter 6 is not required. Thisvariant is not optimal because the sensor 3 also must keep informationon its individual calibration. Therefore memory circuitry would still beneeded and the reusable module would also need to read the memory toretrieve the calibration data (or otherwise the processor 7 would needto obtain the calibration data), or the sensor 3 would still need tohave a processor to calibrate its signal so that the flow data wasadjusted by the calibration data. This can increase the cost of thedisposable tube 1.

In the embodiment of FIG. 1C, the proximal end of the tube 1 has atongue-depressor extension 1 a that can be positioned to rest on theuser's tongue so that the tongue does not close part of the flow tubeopening altering air flow measurements and thus airway resistancemeasurement results.

Baffle for Improved Flow Measurement Response

FIG. 2 a shows experimental Venturi-type flow tube 1 with one pneumaticport 4 connected to the gauge sensor 3. The tube was tested withoutbacteriological filter. One tube tested had a length of 80 mm with aninput diameter of 15.5 mm and diameter in the narrowest part of 13 mm.It will be appreciated that variations in dimensions and shape arepossible.

The flow tube was characterized without baffle and with baffle 9 havingheight of 1 mm. FIG. 3 shows experimentally measured pressure responsein both cases. Generated pressure (negative with respect to ambient) forthe flow tube with baffle exceeds pressure for no-baffle tube by factorof ˜2 for medium flows and ˜5 for low flows.

The arrangement of a baffle in a flow tube is known per se from PCTpatent publication WO01/18496 A2 published 15 Mar. 2001 (see for exampleFIG. 8A).

In both cases of baffle and no baffle, the generated pressure is closeto the square function of flow. FIG. 4 shows calibration curves of bothtubes presented with axes “flow” and “square root of pressure”. Notethat in real operation, flow through the flowmeter is restored frompressure (measured by gauge sensor 3). Pressure response of the flowtube with baffle provides more accurate flow measurement and requiressimpler linearization procedure because flow is almost linear functionof “square root of pressure”. Alternatively, the flow tube withoutbaffle requires more complicated linearization due to essentialdeviation of calibration curve from linear function of “square root ofpressure” at low flows.

It was experimentally found that there is optimal value of the baffleheight for the tube with certain geometry. FIG. 5 shows calibrationcurves of the second tube with a length of 82 mm, input diameter of 19mm and diameter in the narrowest part of 14 mm. The tube without bafflehas the lowest sensitivity and essential deviation from pure “squareroot of pressure” function at low flows. Sensitivity here is treated asability of the tube to generate entrainment pressure as function offlow. Sensitivity of the tube with the baffle of intermediate height of1.5 mm is higher than for the no-baffle tube but also higher than forthe tube with bigger baffle with 2.2 mm height.

Additional advantage of the baffle with intermediate height is thatpressure noise caused by flow turbulence is lower than for the biggerbaffle. FIG. 6 shows noise measured as percentage of measuredentrainment pressure. Noise for the tube with 2.2 mm baffle is aboutfour times bigger than for the tube with optimal (1.5 mm) baffle.

It is assumed that for the flow tube of Venturi type generatingentrainment pressure, baffle with optimal geometry (height) must befound. Tube design optimization may include experimentalcharacterisation of the tubes with different baffles and experimentalselection of the baffle design which provides the highest sensitivitywith optimal pressure noise. Alternatively theoretical simulation of theflow tube can be done to determine optimal design.

FIG. 2 b shows another embodiment of the Venturi-type flow tube 1 withbaffle 9. This tube contains additional pneumatic port 13 withdifferential pressure sensor 12 connected to the ports 4 and 13.

Gauge sensor 3 operates as was described above and measures pressureduring occlusion stage and post-occlusion flow spike. Data from thesensor 3 are used for calculation of airway resistance. Sensor 12 can beused to determine direction of flow through the tube 1. With thisflowmeter, it is possible to provide multiple airway resistancemeasurements during spontaneous breathing. After occlusion stage andpost-occlusion flow spike, the subject continues spontaneous breathingthrough the flow tube. Sensor 12 together with sensor 3 can determineend of inspiration and generate signal for shutter closing. Airwayresistance measurements can be repeated automatically multiple times atthe beginning of exhalation.

The flow tube with baffle having optimized sensitivity and noise can bemade in a non-disposable form in accordance with an embodiment similarto FIG. 1 .

It should be noted that additional technical solutions may be used bythose skilled in the art to improve and extend some features of thedevice. For example the flow tube can be attached to specially designedmouthpiece to provide optimal and comfortable usage by the subject.Alternatively the proximal end of the flow tube can be specially shapedto better fit mouth of the subject.

What is claimed is:
 1. A device for measuring airway resistancecomprising: a shutter; a flow tube having a proximal end, for engaging apatient's mouth, a distal end, for receiving said shutter to releasablyocclude said flow tube, and a medial pneumatic port; a baffle locatedupstream of said pneumatic port, said baffle projecting from a sidewallnear said pneumatic port so as to create a negative pressure at saidpneumatic port; a sensor for measuring pressure in said flow tube duringocclusion and entrainment pressure caused by air flow through the tubeduring exhalation, wherein said sensor is in communication with saidpneumatic port; and a shutter release mechanism to keep said flow tubeclosed with said shutter during occlusion and to release said shutter atthe end of occlusion.
 2. The device as defined in claim 1 wherein saidsensor is a calorimetric thermal flow sensor having two ports, a firstport being in communication with said medial pneumatic port and a secondport being in communication with ambient air.
 3. The device as definedin claim 1, further comprising a bacteriological filter between saidmedial pneumatic port and said sensor.
 4. A device for measuring airwayresistance comprising: a shutter; a flow tube having a proximal end, forengaging a patient's mouth, a distal end, for receiving said shutter toreleasably occlude said flow tube, and a medial pneumatic port; a bafflelocated upstream of said pneumatic port, said baffle projecting from asidewall near said pneumatic port so as to create a negative pressure atsaid pneumatic port; a sensor for measuring pressure in said flow tubeduring occlusion and entrainment pressure caused by air flow through thetube during exhalation, wherein said sensor is in communication withsaid pneumatic port; and a shutter release mechanism to keep said flowtube closed with said shutter during occlusion and to release saidshutter at the end of occlusion, wherein said baffle has a shapedependent on flow tube geometry and the shape is further dependent onmodifying an output of a combination of pressure value, pressure noise,and nonlinearity of pressure response versus flow.
 5. The device asdefined in claim 4 wherein a height of the baffle is selected to reach amaximum entrainment pressure as a function of flow during quietexhalation.
 6. The device as defined in claim 4 wherein a height of thebaffle is selected to reduce pressure noise due to flow turbulence. 7.The device as defined in claim 4, wherein said sensor is a calorimetricthermal flow sensor having two ports, a first port being incommunication with said medial pneumatic port and a second port being incommunication with ambient air.
 8. The device as defined in claim 4,further comprising a bacteriological filter between said medialpneumatic port and said sensor.